In recent years, in the automobile industry, due to growing awareness of environmental issues, exhaust gas regulations have been further tightened and measures for decreasing carbon dioxide emission have been taken. In addition to the measures from a fuel standpoint such as bioethanol and biodiesel fuels, measures for further decreasing a vehicle weight and installing an exhaust gas treatment device such as EGR, DPF (Diesel Particulate Filter) and a urea SCR (Selective Catalytic Reduction) system have been taken. Further, in order to improve a fuel efficiency, exhaust heat recovery equipment to recover exhaust heat has begun to be installed.
Among these, an object of the EGR cooler is to lower a combustion temperature and decrease NOx, which is a poisonous gas, by cooling engine exhaust gas with an engine cooling water and subsequently returning the exhaust gas to an intake side for recombustion. The exhaust heat recovery equipment is a system to heat the engine cooling water with the exhaust gas to use for warm-up of a heater and the engine. The exhaust heat recovery equipment is also called an exhaust heat recirculation system. This arrangement shortens a time between a cold start to an engine stop in a hybrid car, and contributes to an improvement in a fuel efficiency especially in winter.
Further, also in the field of the hot water supplier, application of the heat exchanger has been increased according to a widespread use of an environment-responsive device. In a gas hot-water supplier, a latent heat recovery-type gas hot-water supplier added with a secondary heat exchanger made of a stainless steel has been widespread in order to recover a latent heat from an exhaust gas having a high temperature approximately from 150 degrees C. to 200 degrees C., the exhaust gas having been usually discharged without any treatment. Moreover, a typical electric hot water supplier including a built-in heater has been increasingly switched to a CO2 refrigerant heat pump-type hot water supplier (trading as EcoCute (registered trademark) which can decrease electric energy to ⅓ or less. A heat exchanger is also used in the CO2 refrigerant heat pump-type hot water supplier.
Such a heat exchanger is required to have a favorable heat conductivity for a favorable heat efficiency, and an excellent corrosion resistance against exhaust gas condensate water since the heat exchanger is in contact with an exhaust gas. In automobile components, the EGR cooler and the exhaust heat recovery equipment are required to have much higher safety and more excellent corrosion resistance since leakage of a cooling water is likely to cause a critical breakage in the EGR cooler and the exhaust heat recovery equipment.
An austenitic stainless steel (e.g., SUS304 and SUS316L) with corrosion resistance and strength is generally used as a material for the heat exchanger.
Some heat exchangers are assembled by welding and others are assembled by brazing because of a complicated structure of the heat exchanger. A material for a heat exchanging portion to be assembled by brazing is required to have a favorable brazeability.
Patent Literature 1 discloses a brazing process of coating a phosphorus-containing nickel alloy by electroless plating onto a surface of a component of a heat exchanger made of a stainless steel material, and subsequently melting the phosphorus-containing nickel alloy in high temperature vacuum to use the molten substance as a brazing material. The usable stainless steel is exemplified by SUS304.
Patent Literature 2 discloses a cylindrical structural body usable as a brazed member made of an austenitic stainless steel, the cylindrical structural body being a part of an engine exhaust gas purifying device and housing a metal support supporting an exhaust gas purifying catalyst thereon. Patent Literature 3 discloses a common rail used for a low pressure fuel. Neither Patent Literature 2 nor Patent Literature 3 discloses a type of steels. Similarly, Patent Literature 4 discloses a heat exchanger pipe for a heat exchanger of an EGR gas cooler. Examples of an austenitic stainless steel used for a wavy fin structural body of the heat exchanger pipe include SUS304, SUS304L, SUS316 and SUS316L.
Patent Literature 5 discloses a composite material used for brazing, which is obtained by forming a brazing material layer having a multi-layer structure, in which a layer of Fe or Fe alloy, a layer of Ti or Ti alloy, and a layer of Ni or Ni alloy are layered, on a surface of a base material made of an alloy containing Ni. Herein, the base material made of an alloy containing Ni is exemplified by an austenitic stainless steel (e.g., SUS304) and a dual phase stainless steel.
Patent Literature 6 discloses an austenitic stainless steel having excellent corrosion resistance and brazeability and containing C: 0.080% or less, Si: from 1.2% to 3.0%, Mn: from 0.4% to 2.0%, P: 0.03% or less, S: 0.003% or less, Ni: from 6.0% to 12.0%, Cr: from 16.0% to 20.0%, Cu: from 0.2% to 3.0%, Mo: from 0.1% to 1.0%, Al: from 0.002% to 0.10%, and N: from 0.030% to 0.150%, so as to satisfy a relationship of 1.6≤Cu×Si≤4.4 and a relationship of 0.16≤2N+Mo≤1.0.
Patent Literature 7 discloses an austenitic stainless steel usable as member for an exhaust gas flow path, the austenitic stainless steel containing C: from 0.001% to 0.03%, Si: from 0.10% to 0.70%, Mn: from 0.10% to 1.00%, P: from 0.005% to 0.045%, S: 0.003% or less, Ni: from 18.00% to 40.00%, Cr: from 20.00% to 30.00%, Cu: 2.00% or less, Mo: from 3.00% to 8.00%, Al: 0.13% or less, and N: from 0.05% to 0.30% so as to satisfy a relationship of Cr+2Mo+0.5Ni≥40.
Patent Literature 8 discloses a ferritic stainless steel suitable for a heat exchanger member with Ni-braze or Cu-braze, the ferritic stainless steel containing C: 0.03% or less, Si: 3% or less, Mn: 2% or less, P: 0.005% or less, S: 0.03% or less, Cr: from 11% to 30%, Nb: from 0.15% to 0.8%, N: 0.03% or less so as to satisfy a relationship of Nb−(C×92.9/12+N×92.9/14)≥0.10.
Patent Literature 9 discloses a ferritic stainless steel material used for brazing, the ferritic stainless steel material containing C: 0.03% or less, Si: from more than 0.1% to 3%, Mn: from 0.1% to 2%, Cr: from 10% to 35%, Nb: from 0.2% to 0.8%, and N: 0.03% or less, and having a partially recrystallization structure with from 10% to 80% of an area ratio of recrystallization particles generated by heating after cold working.